Octoechos

Octoechos (from Greek οκτοηχος) can refer either to the eight-tone (or mode) system of Church music or to the liturgical book containing the weekly variable texts in each of the eight tones. St. John of Damascus is credited with the systemization of the musical forms of the Church.

In Greek usage, the Octoechos book is only the Resurrectional material for Sunday services which varies in the eight-week cycle, and thus is also called the Anastasimatarion. The Greek book for all seven days of the weekly material is the Parakletiki. In Slavic usage, the Octoechos (or Oktoich) book includes the material for every day of the week, and thus is equivalent to the Greek Parakletiki.